1. Field of the Invention
The present invention relates generally to a beam compass used in drafting.
2. Description of the Prior Art
Many types of beam compasses have been employed in the past. A typical beam compass includes a center pivot, a marking instrument (pen or pencil) holder and a beam connecting the marking instrument holder and the center pivot. The radius of the arc to be drawn may be set by adjusting the distance between the marking instrument holder and the center pivot along the beam.
When using a beam compass, it is frequently necessary to use marking instruments having points of varying hardness and thickness. However, many beam compasses, such as U.S. Pat. No. 3,002,280 to Bennett, Jr., U.S. Pat. No. 2,943,392 to Attridge and U.S. Pat. No. 3,797,118 to Yamamoto, do not provide a marking instrument holder, through which the marking pen or pencil projects, designed for marking instruments having points of different sizes.
U.S. Pat. No. 3,513,548 to Itano discloses the use of apertures of varying diameters to accomodate marking instrument points of different sizes or thicknesses. However, Itano provides these apertures along the longitudinal axis of the beam. Thus, it is necessary to readjust the position of the marking instrument holder relative to the center pivot, to trace the original arc through the different apertures. This realignment of the desired aperture with the desired arc radius must be performed prior to using the new marking instrument. These adjustments can be very time consuming and tedious.
With prior art beam compasses, it has also been difficult to properly adjust the compass to draw an arc or circle of desired radius. Because those marking instrument holders literally cover the desired point of contact on a drawing surface, it is difficult for the draftsman to locate the center pivot. Thus, the draftsman may have to align the marking instrument by trial and error. This can prove disastrous if a mark is incorrectly made with ink or other indelible material.